pc-bios: Update palcode-clipper

Update image to c87a92639b28ac42bc8f6c67443543b405dc479b,
incorporating changes for vm_time.

Signed-off-by: Richard Henderson <rth@twiddle.net>
diff --git a/pc-bios/README b/pc-bios/README
index 030d92a..53b5289 100644
--- a/pc-bios/README
+++ b/pc-bios/README
@@ -40,4 +40,4 @@
   git://repo.or.cz/s390-tools.git
 
 - The sources for the Alpha palcode image is available from:
-  git://repo.or.cz/qemu-palcode.git
+  git://github.com/rth7680/qemu-palcode.git